1. Field of the Invention
The present invention relates to an electronic ballast device for at least one fluorescent lamp equipped with heatable electrodes, the device comprising a D.C.-A.C. converter having an input for receiving an operating D.C. voltage and an A.C. output and a D.C. output connected to a load which comprises a fluorescent lamp, the lamp heater electrodes, and a transformer for the potential-free connection of the fluorescent lamp to the output of the D.C.-A.C. converter.
2. Description of the Prior Art
Given a turn-on event with a fluorescent lamp equipped with heatable electrodes, the electrodes constructed as heating coils and referred to below in brief as heating or heater electrodes, must first be adequately preheated in order to be able to subsequently ignite the fluorescent lamp reliably and free of flickering effects.
For example, U.S. Pat. No. 3,710,177, fully incorporated herein by this reference, produces a high-frequency heating of the heating electrodes with the assistance of a frequency shift of the switching frequency of the D.C.-A.C. converter. The task of the frequency shift is, first of all, to assure a rapid preheating and, secondly, to prevent the fluorescent lamp from receiving a high voltage during the preheating interval that would prematurely effect its ignition.
As warranted, the voltage at the fluorescent lamp can also be lowered during the preheating interval simultaneously with the frequency shift.
Since the high-frequency performance of the D.C.-A.C. converter is limited, the preheating time cannot be kept arbitrarily short. As a rule, preheating times on the order of magnitude of more than 1.2 seconds thereby result. In other words, the turn-on of the fluorescent lamp is delayed by the preheating time over a time interval that definitely proves disturbing.